For a block copolymer comprising a conjugated diene and a vinyl aromatic hydrocarbon, in the case that the vinyl aromatic hydrocarbon content is relatively low, then even if not vulcanized, the block copolymer has a similar elasticity at ordinary temperature to a vulcanized natural rubber or synthetic rubber, and moreover at high temperature has a processability similar to a thermoplastic resin; such block copolymers are thus widely used in fields such as footwear, plastic modification, asphalt modification, and adhesives. On the other hand, in the case that the vinyl aromatic hydrocarbon content is relatively high, a thermoplastic resin that is transparent and has excellent impact resistance is obtained; such block copolymers are thus used for food packaging containers, household goods, packaging materials for household electrical equipment or industrial parts, toys, and so on. Furthermore, hydrogenated products of such copolymers have excellent weather resistance and heat resistance, and hence are widely used in the above fields of application, and also for automobile parts, medical equipment, and so on.
However, a block copolymer having a relatively low vinyl aromatic hydrocarbon content, despite having good flexibility, has the drawback that the abrasion resistance is poor, and hence there are constraints in terms of further expanding the applications thereof. On the other hand, conventional block copolymers having a relatively high vinyl aromatic hydrocarbon content have poor flexibility, and hence have not been suitable as soft materials.
As a flexible material, there has been disclosed a composition comprising a polypropylene resin, and a hydrogenated diene copolymer obtained by hydrogenating a copolymer that is a random copolymer having a vinyl aromatic hydrocarbon content of from 3 to 50 wt %, has a molecular weight distribution (Mw/Mn) of not more than 10, and has a vinyl bond content in the diene portion of the copolymer in a range of from 10 to 90% (see, for example, Patent Document 1: Japanese Patent Application Laid-open No. 2-158643). Moreover, there has also been disclosed a composition comprising a polypropylene resin, and a hydrogenated diene copolymer obtained by hydrogenating a copolymer that is a random copolymer having a vinyl aromatic hydrocarbon content of from 5 to 60 wt %, and has a vinyl bond content in the diene portion of the copolymer of not less than 60% (see, for example, Patent Document 2: Japanese Patent Application Laid-open No. 6-287365).
There have been attempts to use such hydrogenated diene copolymers in applications in which soft vinyl chloride resins have been conventionally used. For soft vinyl chloride resins, there are worries over problems with the halogen upon burning, the environmental problem of environmental hormones due to plasticizers, and so on, and hence the development of alternative materials is an urgent task. However, hydrogenated diene copolymers such as the above have not had adequate properties in terms of flexibility and abrasion resistance to be used in applications in which soft vinyl chloride resins have been conventionally used.
In recent years, from among block copolymers having a relatively high vinyl aromatic hydrocarbon content, there have been attempts to obtain flexible materials, and a molding material having as its base a hydrogenated block copolymer comprising a copolymer containing a block composed mainly of styrene and a block composed mainly of butadiene/styrene (see, for example, Patent Document 3: WO 98/12240) has been disclosed. However, the hydrogenated copolymer disclosed therein is lacking in flexibility, and hence has not been suitable for applications in which a soft vinyl chloride resin has been used.
In this way, even though there is an urgent need to develop materials as alternatives to soft vinyl chloride resins for which there are various environmental problems, the current state of affairs is that a material having properties (flexibility, abrasion resistance, etc.) comparable with those of a soft vinyl chloride resin has not been obtained.
The present inventors have carried out development of alternative materials to soft vinyl chloride resins, and have previously developed a hydrogenated copolymer having flexibility and abrasion resistance comparable to those of soft vinyl chloride resins (see, for example, Patent Document 4: WO 03/35705). However, the emergence of a material having better heat resistance than that copolymer has been desired.